Measurement engineering is a science dealing with sensor apparatuses for sensing a physical value and converting the sensed physical value into an electronic measurement signal, which can be used for further signal processing.
Such a sensing apparatus usually includes a sensing device and a preprocessing device.
The sensing device is named sensor head. It measures the physical value and converts the measured physical value into an electronic sensor signal. This sensor signal is usually weak and includes a lot of unintended signal components like noise components and an offset component. These unintended components make a direct signal processing with the sensor signal unsuitable.
To prepare the sensor signal for further signal processing, the preprocessing device generates the measurement signal including only signal components of the sensor signal having information about the measured physical value. For simplification, these signal components should be named interesting components. The preprocessing device thus removes the unintended components of the sensor signal and amplifies the interesting components.
For removing the unintended components, different approaches have been proposed. The common idea of all of these solutions is to determine the unintended components and to remove these determined components by special filters. Two different examples should illustrate the procedure. The offset component may be determined by superimposing the sensor signal with a sensor signal being shifted by a shifting angle of 180 degrees. The determined offset component can then be subtracted from the sensor signal to generate the measurement signal. However, the shifting angle is technically realized by a time delay, which is subjected to technical errors. Due to these technical errors, not only the offset component will be removed but the interesting components will also be distorted.
Alternatively, a high pass filter may be applied to remove the offset component. Although the high pass filter removes the offset, it also distorts the sensor signal around 0 Hz, since there exist no high pass filter having an ideal cut-off frequency for removing only signal components having 0 Hz.
As can be seen from the forgoing example, when removing unintended signal components from the sensor signal, the remaining interesting signal components would forcibly be distorted. This increases the measurement error of the measured physical value and complicates the whole signal procession of the measurement signal output by the sensing apparatus.
To attenuate the effects of the distortions of the interesting signal components of the sensor signal, further technical means must be provided. However, these technical means would counteract another important technical requirement of the sensing apparatus—the limited space for its implementation. Modern electronic devices tend toward miniaturization, such that available space for sensing apparatuses as described above becomes smaller. This trend does not allow providing a complex preprocessing device for attenuating negative effects, which artificially occur due to the technical implementation of the sensing apparatus. In other words, it would be better to avoid distortions of the interesting components than to process the distorted interesting components afterwards.